In this paper, environmental and geographical subject expertise is explored, based on the notion of expertise as rooted both in subject knowledge and knowledge of teaching and as acquired through experience. The research questions that guided this study focused on the importance of understanding how teachers negotiate their assumptions and what sources of knowledge support their claims. As such, teachers' knowledge was investigated as a step towards the professionalisation of their practices. In terms of methodology, 21 teachers were interviewed in the context of a larger research project currently being conducted on Chilean teachers' thought processes. Semi-structured interviews were designed to elicit data on teachers' practices and conceptualisations. Data analysis involved codification of these same practices and conceptualisations, and defining categories that encapsulated teachers' expertise. The key findings from analysing teacher narratives included the observation that their practices and notions of subject expertise were identified as experience-based. Teachers' uncertified expertise in areas where they had had no formal training could be understood as mechanisms they have developed during their professional career. These elements could be seen as an implicit self-generated supporting system for teachers' practices.